This invention relates to non-contact type strain detectors for detecting strains developed by the torque applied to a shaft of rotating machines, etc., on the basis of the magnetostrictive effect without direct contact with the shaft.
Recently, proposals have been made of non-contact type strain detectors (also called torque transducers or torque sensors) utilizing stress- or strain-senstive ribbon-shaped thin elements attached to a shaft in a chevron-pattern. FIG. 1 shows an example of such a strain detector, which is disclosed, for example, in Japanese Published Unexamined Patent Application No. 57-211030. As shown in FIG. 1, on the side surface of a driven shaft 1 to which a torque is applied are attached a plurality of magnetostrictive elements 2 and 3 in a chevron pattern. The elements are made of a soft magnetic material having a permeability that varies in accordance with the magnitude of the strain developed therein by the torque applied to the shaft 1. The strain-sensitive elements are arranged in two parallel groups of ribbons 2 and 3 oriented at +45 and 31 45 degrees, respectively, with respect to the axis of the shaft 1. Annular detection coils 4 and 5 are wound around the two groups of magnetostrictive strain-sensitive elements 2 and 3, respectively, for detecting variations in the permeability thereof.
The method of operation of the strain detector of FIG. 1 is as follows. When a torque is applied to the shaft 1, principal stresses are generated in the shaft 1 and hence in the elements 2 and 3 attached thereto along principal axes having directions which agree with the longitudinal directions of the two groups of magnetostrictive elements 2 and 3, respectively. These principal stresses act as a tensile force on one of the two groups 2 and 3 of magnetostrictive elements, and as a compressive force on the other of the two groups. This causes variations in the permeabilities of these magnetostrictive elements 2 and 3 as follows. Generally, when a stress is applied to a magnetic material, its magnetic properties are changed to vary its permeability (i.e., the permeability increases or decreases depending on the case). This effect, corresponding to the Villari effect by which the permeability of a magnetic material is changed in accordance with the deformation or strain developed therein, is utilized in magnetostriction transducers which convert mechanical energy into electrical energy. The constant which represents quantitatively the magnitude of the magnetostrictive response of a magnetic material is called the magnetostrictive constant. The magnetostrictive constant of a material represents quantitatively the dependance of its permeability or susceptibility on the strain developed therein. When the value of the magnetostrictive constant is positive, the permeability increases under a tensile force; when its value is negative, the effect is reversed. Thus, the permeabilities of the two groups of elements 2 and 3 are increased or decreased in accordance with the deformation generated therein corresponding to the amount of torque applied to the shaft 1. These variations in the permeabilities of the two groups of elements 2 and 3 are detected by coils 4 and 5, respectively, as variations in the magnetic impedance. Thus, the strain in the shaft 1 can be detected without direct contact therewith.
These non-contact type strain detectors are excellent in principle; however, there still remains room for their improvement. Thus, many proposals have been made for the improvements of strain detectors of the above type.
For example, Japanese Published Unexamined Patent Application No. 59-180338 teaches a torque sensor in which a magnetic core made of an amorphous alloy is utilized in the detection of the variation of magnetic characteristics. Further, Japanese Published Unexamined Patent Application No. 60-260821 discloses a torque sensor in which a yoke made of a copper-nickel type amorphous alloy is provided for preventing magnetic flux leakage to the exterior of the detection coil.
However, these proposals for the improvement of non-contact type magnetostrictive strain sensors are not sufficient. Namely, since the Curie temperature of amorphous magnetic materials is low, the magnetic properties thereof tend to vary over time. Further, bands of these materials exhibit anisotropy in the longitudinal direction due to the production process thereof. As a result, when they are disposed as a magnetic yoke around a shaft in the form of multiple layers, anisotropy perpendicular to the direction of the magnetic flux is inevitable; although improvements thereof by means of heat treatments in magnetic fields have been attempted, such treatments are accompanied by handling difficulties caused by the brittleness of the material and can not ensure secure adhession of the material. Further, the disadvantage of the amorphous magnetic alloys utilized as the material of the magnetostrictive elements is that the magnetostrictive constants thereof are not sufficiently great to guarantee high enough sensitivity of the strain detector. In addition, when they are utilized as the material of the magnetic yoke, noises are generated due to the resonance caused by the magnetostriction.
Furthermore, since the above-mentioned devices comprise no magnetic shields, the prevention of the magnetic flux leakage and the protection against exterior magnetic disturbances have not been enough. In addition, the support structure of the magnetic yokes is not clear, and the magnetic characteristics of the yokes may suffer variation under application of thermal stresses from other members.